The modern telephone and data network depends upon silica fibers for the transmission of data over optical fiber linking the transmitting and receiving ends. Silica fiber, while offering nearly unlimited bandwidth, has some limitations. Although its minimum absorption is centered in a band around 1.5 .mu.m, an important characteristic of a fiber is its frequency-dependent dispersion. The dielectric constant, and thus the propagation speed of a signal on a fiber, varies with the frequency of the light propagating on the fiber. Dispersion is the rate of change of the dielectric constant with respect to wavelength (or frequency). Dispersion in a fiber causes optical signals at different optical frequencies to propagate at different speeds. As a result, short optical pulses, which in view of Fourier analysis contain many optical frequencies, spread in their temporal length as they propagate along a fiber exhibiting dispersion. Dispersional broadening limits the spacing between pulses and hence also limits the digital data rate that a fiber of a given length can support. Dispersional limitations in silica optical fibers can be largely overcome by conveying optical signals on the fiber in an optical frequency band around 1.3 .mu.m because the dispersion passes through zero at 1.31 .mu.m. Hence, the existing fiber networks for local exchange access are optimized for the 1.3 .mu.m band.
The light source is probably the most challenging component requiring development for a fiber communication system. Bellcore has issued Technical Advisory TA-TSY-000983, Issue 1, 1990 entitled "Reliability Assurance Practices for Optoelectronic Devices in Loop Applications," which defines requirements for the laser diode or other equivalent light emitting elements. Aggressive target specifications for the light emitting element are listed in TABLE 1. These requirements are so strict that only a semiconductor laser could realistically satisfy them. In addition, since the application is for a light source in the local telephone loop, the device should be uncooled, that is, no thermoelectric coolers is needed, so that the cost is kept low.
TABLE 1 ______________________________________ Parameter Minimum Maximum Unit ______________________________________ Operating -45 85 .degree.C. Temperature I.sub.Th @ 25.degree. C. 1 20 mA I.sub.Th 3.5 50 mA P.sub.Op @ I.sub.Mod = 25 mA 4.0 mW P.sub.Op @ I.sub.Th 50 .mu.W .DELTA..eta. 25 to 85.degree. C. and .+-.1 dB 25 to -45.degree. C. Mean Wavelength 1270 1340 nm FWHM 5 nm RMS 2.5 nm V.sub.f @ I.sub.Mod = 25 mA 1.5 V Kink Current 60 mA ______________________________________
Except as specified, these requirements apply to the entire stated temperature range.
Some of these requirements are particularly difficult to satisfy, particularly at the higher temperatures. These difficult requirements include: threshold current I.sub.Th at which the diode begins to lase; differential quantum efficiency .eta. and its change .DELTA..eta.; and the optical output power P.sub.Op at the operating current I.sub.Mod. The operational temperature range extends to 85.degree. C. so that expensive and unreliable thermoelectric coolers are not required.